giving back the pawn to activate the rook.
25. f2f3 This endgame is deceptive. For now Black is a pawn up, but it's clear that he will not hold a material advantage for long. The fact that White has a big advantage may come as a surprise with such reduced material. But the difference in activity between the pieces (including the king) is quite big. 25... b7b5  26. a2a3 This allows Black excellent defensive chances. 26... a6a5  27. b4c3 